My $.02-I have done this to several 67-72 trucks. First, replace the clutch if the compressor still turns OK(you can put a socket on the nut in the middle), replace the dryer w/a R134A compatible one(just about any new one), Old Air in Ft Worth makes a kit to replace the Suction Throttling Valve(weird looking gizmo near the evaporator)this changes it to a cycling clutch setup,charge w/R134A & ester oil.
The old long A6 compressor is pretty bulletproof.
